1. Field of the Invention
The invention is directed to an improved electromagnetically actuatable valve for a hydraulic control device for a motor vehicle.
2. Description of the Prior Art
One electromagnetically actuatable valve of the type with which this invention is concerned is known from German Patent Disclosure DE 195 36 619 A1 and is suitable for hydraulic controls in traction-controlled brake systems. For economical mass production, this valve is provided with simple parts by providing that the magnet valve has a hydraulic part with a valve dome, onto the outside of which valve dome the electrical part of the valve with the electromagnet is slipped. To embody this valve for high pressures and fast switching, it is embodied as a pressure-equalized seat valve. For the pressure equilibrium, in this magnet valve, the tappet that carries the actual closing member is hollow and embodied with a continuous longitudinal bore, in which a pin is guided with a relatively narrow gap and thus tightly; the pin diameter and the effective sealing diameter at the closing member are of equal size, in order to achieve the pressure equilibrium. Moreover, over a long axial range, the tappet is guided with its jacket face in a housing bore in such a way that a gap for transmitting pressure from an outflow opening into a pressure chamber of the valve dome is available for the pressure equilibrium of the seat valve. Although this valve is already economical in construction and makes a fast switching time possible, these sealing provisions on the piston principle, with long pistons and bores and with sealing gaps and leakage gaps, mean that the switching times are technologically limited and are therefore unsuitable for some applications that involve especially short switching times. Moreover, this valve is embodied only as a magnet valve that is normally open and does not make a valve construction that is normally closed possible. The pin in the tubular, longitudinally movable tappet is braced against the housing and also makes for a complicated construction, which is moreover less well suited to miniaturization of valves.
From European Patent Disclosure EP 0 720 551 B1, an electromagnetically actuatable valve is also known which by its simple construction is suitable for economical mass production and can be embodied as either a normally closed or a normally open magnet valve of the seat valve type. However, in this valve, there are different ratios of surface area in the hydraulic region, and hence this valve is not pressure-equalized; moreover, it has relatively long sealing provisions based on the piston principle, with a leakage gap between the piston and the bore, so that these valves too are limited in terms of the pressure that can be switched and their switching times to such an extent that they cannot meet especially stringent demands. This is above all true if switching times in the range of a millisecond or less are required.
From German Patent Disclosure DE 38 02 648 A1, an electromagnetically actuated valve is also known, which with a compact construction is embodied as a fast-switching, pressure-equalized seat valve. In this valve, through which the flow can be in both directions, the armature itself forms the movable closing member and is embodied for that purpose as a sleeve, which is disposed longitudinally displaceably on a guide pin that is structurally connected to the housing. Once again, the guide pin operates with axially relatively long seals on the piston principle, or seals with an O-ring are employed. Particularly the O-ring or an elastomer seal, because of their radial contact pressure, cause friction and thus problems that stand in the way of shortening the switching time. Also, this valve has a special structural form, which cannot be derived from valves, produced in large-scale mass-production, for traction-controlled brake systems.